feat: playlist art fix + Play/Shuffle buttons in context header

- Fix playlist art: API returns images/images150/images300 arrays, not
  image_rectangle; update Rust model and Qt header to use images300 →
  images150 → images with fallback
- Add Play (▶) and Shuffle (⇄) buttons to the album/playlist header;
  Play starts from track 1, Shuffle enables shuffle mode and plays from
  a randomised position

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/list/tracks.cpp

@@ -84,6 +84,17 @@ void Tracks::setPlayingTrackId(qint64 id)
     m_model->setPlayingId(id);
 }
 
+void Tracks::playAll(bool shuffle)
+{
+    const QJsonArray tracks = m_model->currentTracksJson();
+    if (tracks.isEmpty()) return;
+    m_queue->setShuffle(shuffle);
+    m_queue->setContext(tracks, 0);
+    const qint64 firstId = static_cast<qint64>(m_queue->current()["id"].toDouble());
+    if (firstId > 0)
+        emit playTrackRequested(firstId);
+}
+
 
 void Tracks::onDoubleClicked(const QModelIndex &index)
 {

src/list/tracks.hpp

@@ -28,6 +28,10 @@ namespace List
         /// Called when the backend fires EV_TRACK_CHANGED so the playing row is highlighted.
         void setPlayingTrackId(qint64 id);
 
+        /// Start playing all tracks in the current view from the beginning.
+        /// If shuffle is true, enables shuffle mode before starting.
+        void playAll(bool shuffle = false);
+
         /// Set which playlist is currently displayed (0 = none).
         /// isOwned controls whether "Remove from this playlist" is shown.
         void setPlaylistContext(qint64 playlistId, bool isOwned = false);